The long range goal of this project is to make a significant contribution to understanding the various chemical mechanisms associated with synaptic transmissiion. Neural circuits involving identified pre-and post-synaptic neurons have been described in different invertebrate preparations where acetylcholine (ACh), 5-hydroxytryptamine (5-HT), histamine (Hm), dopamine (DA), and gamma-aminobutyric acid (GABA) act as neurotransmitters. The studies proposed here will take advantage of the relatively simple anatomy of the nervous system of the marine mollusc, Aplysia california, to further these investigations. In this molusc definition of some of these neurotransmitters (ACh, 5-HT and Hm) has been accomplished through a series of inter-related chemical, electrophysiological and pharmacological studies, all conducted at the level of the individual neuron. DA, GABA and octopamine have been found in Aplysia ganglia, individual neurons containing significant amounts of these substances have not yet been identified. In addition, certain other neural circuits have been described in which non of these substances appears to be the neurotransmitter. We propose to study in depth those neurons containing defined transmitters as well as to characterize the unknown transmitters used by othe neurons. Through these investigations of various transmitter-mediated processes, we hope to gain insights into the nature of these processes in more complex nervous systems where combined chemical, physiological and pharmacological studies of individual neurons are not yet possible.